Introduction
Periodontitis (PD) is a frequent chronic inflammatory disorder with clinical manifestations of radiologically assessed alveolar bone loss, accompanying clinical attachment loss, gingival bleeding, and periodontal pocket formation [
1]. Host immunophysiological disruption and genetic factors play an important role in the development of PD [
2,
3]. Recently, genetic susceptibility factors were found to play a role in the development of PD. For instance, Matthias Munz et al. have identified a correlation between genetic variants in the loci SIGLEC5 and DEFA1A3 and the risk of PD [
4].
Chronic obstructive pulmonary disease (COPD) is a chronic inflammatory disease characterized by restriction of expiratory airflow and persistent lung parenchymal obstruction, along with emphysematous lung destruction [
5]. Changes in lung microecology, autoimmune components of the disease, and environmental risk factors together contribute to the development of COPD. In addition to the above, genetic susceptibility has been implicated in COPD [
6].
Although COPD primarily affects the lungs, it is now considered as a multi-component disease, often co-existing with other disorders such as PD [
7]. Similarities between the two diseases exist, with neutrophil infiltration, altered protease/anti-protease, and redox state balance playing a key role [
8]. Several observational studies show an independent and significant link between PD and COPD. For example, some retrospective studies show that the development of PD increases the risk of death among COPD patients [
9,
10]. Recently, a systematic review including two cross-sectional studies and a case-control study found that frequent COPD exacerbations were strongly associated with PD [
11]. However, in contrast, Arianne K Baldomero and Zhou X, concluded that periodontal health status was not associated with a worsened COPD state [
12,
13]. However, observational studies using cross-sectional or case-control methods suffer from the disadvantages of bias, confounding factors, and reverse causality. Therefore, clarifying the genetic causal relationship between PD and COPD risk is important for assessing the pathogenesis of COPD.
Mendelian randomization (MR) is a causal analysis method that exploits natural randomization in the generation of individual genetic constituents. MR uses genetic data including single nucleotide polymorphisms (SNPs) that are related to exposure, as instrumental variables to assess the causality of the association between exposure or risk factors and the outcome of interest. MR analysis can overcome the above-mentioned limitations of observational studies in causality investigations and can compensate for the disadvantages of randomized controlled trials (RCTs) including high human and material consumption and ethical challenges. A new perspective unlike traditional observational studies and RCTs is presented [
14]. We conducted a two-sample MR study with data from genome-wide association studies (GWAS) to investigate if there is a genetically predicted causal relationship between PD and COPD, thus facilitating the prevention and treatment of these patients.
Discussion
This is the first report of a two-sample MR analysis for determining the genetic causal relationship between PD and COPD. The MR study provided convincing evidence that PD is genetically causally associated with COPD and is an independent risk factor for COPD development.
The first association between dental health and COPD in community-dwelling populations was discovered in a study of 23,808 people by the National Health and Nutrition Examination Survey I (NHANESI) [
27]. Observational studies suggest that acute exacerbation of PD is a key risk factor for COPD progression and is associated with high mortality in COPD patients [
28‐
30]. Frank A Scannapieco and coworkers found that the more severe the periodontal attachment loss, the greater the risk of COPD (odds ratio: 1.35; 95% confidence interval: 1.07–1.71) [
31]. Hayes et al. also found that PD, as measured by alveolar bone loss assessed by periapical radiographs, was an independent risk factor for COPD [
32]. Niamh Kelly and ZeSheng Wu et al. also suggested that poor periodontal health was associated with worsened COPD [
11,
33]. Recently, Liu Shu qin et al. identified a potential genetic crosstalk between PD and COPD [
34]. PD was further identified as an important and independent risk factor for COPD based on a meta-analysis of 14 observational studies [
35]. Finally, A meta-analysis comprising 75 survey studies revealed a significant positive correlation between periodontitis and COPD [
36]. These findings provide preliminary evidence that PD is an important factor that promotes COPD.
The results of this MR study strongly supported a genetically predicted causal relationship between PD and COPD and demonstrated that the former is an important risk factor for the latter. Therefore, the potential pathophysiological relationship between PD and COPD warrants further investigation.
We believe that the causal relationship between these two diseases may be closely related to the following reasons. First, oral flora is an important factor in the causal relationship between PD and COPD. Previous studies implicated oral bacteria in lung infections. Through various masticatory motions, dental plaque shed into saliva may change the respiratory epithelium, allowing pulmonary pathogen colonies to adhere strongly and grow [
37]. Andreea C Didilescu et al. showed that dental plaque may serve as a reservoir for respiratory bacteria [
38], crucial for the advancement of COPD. Studies have identified common pathogens between PD and COPD, including
Porphyromonas gingivalis,
Tannerella forsythia, Haemophilus, and
Treponema denticola [
39]. Among these,
Porphyromonas gingivalis is closely linked to the development and progression of periodontal disease, being a gram-negative anaerobic bacterium commonly colonizing periodontal pockets [
40]. Nan Feng et al. found that it could migrate to the lungs, alter the pulmonary microbiota, and exacerbate chronic obstructive pulmonary disease (COPD) [
41]. Improved dental health may reasonably lower morbidity in COPD patients [
42]. Second, both diseases are also associated with inflammatory mediators. Local inflammation in the periodontium results in the release of several proinflammatory cytokines into the bloodstream, including interleukins IL-6, IL-1α, and IL-1β, interferon IFN-γ, and tumor necrosis factor (TNF-α) [
43]. These inflammatory factors may be connected to respiratory disease infections. In animal and cell studies, neutrophils have been linked to the onset and progression of COPD, by releasing inflammatory mediators like neutrophil elastase and matrix metalloproteinases (MMPs) [
10]. According to a review by Hajishengallis, diseases characterized by aberrant neutrophil functions have a higher associated frequency of PD, including neutrophil deficiency and autoimmune neutropenia [
38]. Epidemiological studies have found evidence to support this relationship. Finally, it is noteworthy that Kaixin Xiong et al. first discovered the crucial role of the γδ T-M2 immune mechanism in mediating periodontitis-promoted COPD [
44]. Additionally, Shuqin Liu et al. identified EPB41L4A-AS1 as a potential cross-interacting gene between the two diseases. Its downregulation activates the nuclear factor kappa B (NF-κB) signaling pathway and enhances inflammatory responses, which also plays a pivotal role in the pathogenesis of periodontitis-promoted COPD [
34].
Our MR analysis has numerous advantages. First, environmental factors and behavior do not affect genetic variation. As a result, MR analysis decreases residual confounders and other biases, effectively reducing reverse causality [
14]. To evaluate the possible genetic causal link between PD and COPD, we used the most recent and largest GWAS dataset for PD and COPD from a European-descent population, therefore minimizing the influence of population stratification. Additionally, MR also has the advantages of being cost-effective and having few ethical concerns [
45]. Lastly, we used a two-sample design to estimate the link between genetic variant exposure and genetic variant outcomes from two independent comparable populations, yielding greater statistical power [
46].
Limitations
Some limitations also warrant consideration. First, the SNPs used were all from individuals of European ancestry. Such studies for other ethnicities are warranted. Currently, MR studies often assess the lifelong effects of risk factors on outcomes, and it is difficult to reveal the causal effects across the stages of disease development. Finally, given the varied definitions of PD employed in different studies, GWAS for consistent SNPs in PD is difficult.
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